Thorium (Th-232) can be bred to fissile uranium 233 (U-233) by hitting it with a neutron, allowing the Th-233 to decay to protactinium (Pa-233) and then to uranium.

Separation of pure U-233 requires not just a liquid-fuel reactor, but either something called a "two-fluid reactor", or chemical separation of Pa-233. Pa-233 is a strong neutron absorber and will go to Pa-234, which decays to U-234 which is not fissile (it takes yet another neutron to make U-235, which is the natural fissile isotope).

The likelihood of Pa-233 grabbing a neutron before it beta-decays to U-233 is proportional to the neutron flux, so reactors with low power density don't need separation of the breeding and power sections. They can do it all with one uniform fluid.

In a one-fluid reactor, U-233 sometimes gets hit with a neutron and, instead of fissioning, loses another neutron. This is the (n, 2n) reaction and it makes another isotope of uranium, U-232.

U-232 is not fissionable, but another neutron turns it back to U-233.

U-232 has a decay path which creates thallium 208 (Tl-208). Tl-208 is a very powerful gamma emitter, which will degrade explosives and fry electronics unless they are behind heavy shields.

Because U-232 cannot be separated from U-233 by chemical means, any uranium removed from a one-fluid thorium reactor will soon have a growing amount of Tl-208 frying anything nearby and shining like a beacon saying "Here I am!". You can remove the thallium chemically, but as long as there's still U-232 you'll have more as soon as you stop.

A bomb built to fit on a missile can't have heavy shielding, and a "stealth bomb" to be sneaked into an enemy city isn't going to work if its core can be detected though a foot of lead.

If that's not good enough, there's a scheme called a "denatured molten-salt reactor" (DMSR) which has a mix of stuff that's even more useless for bombs and doesn't even breed to breakeven. You have to add small amounts of enriched fuel to keep it going (but you can run 20 years or so without removing anything, making it cheap and trouble-free).

This isn't a problem with solid fuels. The thorium-uranium rods being developed by Lightbridge are even more of a headache for proliferators than straight uranium.

That's why nobody's ever tried to base a weapons program on thorium; if it was so easy, Kim Il Sung, A.Q. Khan and Saddam Hussein would have gone that way. Even the USA realized that neither a weapons program nor a plutonium economy could come from thorium reactors (which we now know is a good thing). Nobody did because they know more about nuclear technology than Steven Den Beste.

Interesting article and interesting technology. I know a little chemistry but do not know enough nuclear science to really comment.

However, I've seen a number of blog-rants _against_ nuclear energy and as I scan them, I do not even find any possible pseudo-facts. Just the empty bloviation.

I have come around to believing that we are doomed, not so much by a lack of technical solutions as the idiocracy we now live in. If, for example, it were feasible to put a little power station in everyone's garage (or on their roof), it would be actively discouraged because it wouldn't enhance the power of any central political authority nor boost the ego or bank account of any of the 'management' class.

And I can see that dynamic working on a large scale, for nuclear technology which does not lead to weapon material.

U234 does fission - if the spectrum is VERY fast. Pa also fissions a bit more in a very fast spectrum. More U232 is made too, since n,2n gets much bigger with faster spectra.

This is possible with a chlorides MSR. Recently, I've been interested in a single fluid once through chloride DMSR started up with transuranic gunk. No online processing required, with a 95+ percent conversion of transuranic waste to useful U233 at the end of the cycle. EP, you really should come comment some more on Energy From Thorium Forum, if you have the time...

Not true. Whilst interned at Farm Hall on 14 August 1945 where conversations were secretly recorded by British captors, Heisenberg revealed that the Nazis during WW2 created a project to build an atomic weapon with Protoactinium (aka procatinum). Heisenberg worked closely with von Laue and Dallenbach at a research institute called Forschungstelle D which was devoted to creation of a heavy particle accelerator (termed "Zyklotron") for breeding Uranium 233. Whilst the weapon contemplated was referred to as a Proactinum bomb, OSS informant Respondek had access in 1942-43 to information about the funding intentions for Forschungsstelle D which clearly identified the aim as a Uranium bomb. Nor was this a dirty bomb. All reference to the Nazi Proactinum bomb has been expunged from the official record of WW2 history, yet there are surviving references which remain overlooked.

Suppose it's true that the Nazis attempted to use accelerator-driven transmutation ("zyklotron" is a cyclotron) to make a U-233 bomb. It would still be impossible to make a reproducible and storable weapon from U-233 generated in a solid-fuel thorium LWR or a single-fluid thorium MSR (without Pa separation). There would be far too much U-232 in the products to make a bomb that could be used much later; the decay to Tl-208 proceeds inexorably and irradiates everything nearby, and neither explosive lenses nor control electronics can be shielded very much in a missile warhead.